Add theme templates for Adminterface

[richchw]

Problem:

One of Adminterface's strengths is the ability to customize theming quickly using Bootstrap styling. This would provide developers & users with the ease of accessibilities and flexibility to design their project to fit their client's branding and/or prefer styling.

Based on the team's internal discussion and feedback from beta users, we have concluded that Adminterface should provide a starter template or stock template specifically for themes so that developers & users can easily replace the theme and focus on building the core functionalities.

Our decision is based on the fact that many contractors or small teams do not have a dedicated designer and they shouldn't have to put so much effort into figuring out how theming/styling should be in the early stages of their project.

Proposed Solution:

Adminterface's theming and styling customization is generally done by editing these 3 files:

• css_classes.yml
• components.yml
• Importing custom SCSS file

Our initial approach will be providing a couple of general themes & styling based on trendy choices from the community and research. For example, some of the long-term themes used by many popular applications such as VS Code or Iterm.

How it works:

1. Developers & users can browse through the "Adminterface Theme Showcase" page to select a desirable theme for their project.
2. Download or copy the files that are required for the theme to replace it within their project.
3. Developers & users can further edit the theme any way they like.

The theming files will be structured as closely as possible based on Adminterface's Docs example so that developers & users can easily refer back to the documentation and understand it.

Showcase Page:

It will contain:

• preview of the theme in gallery mode
• download/copy files
• instructions on how to install themes

Our goal is to:

• minimize the amount of effort it takes to initiate a project without adding too many complications
• ease the amount of work for developers & users so that they could focus on the core functionalities
• keep the theme and styling customization flexible, easily accessible, and well-structured

No Gos:

• We will not be considering other downloading methods for themes in this phase.